Simple shear is not so simple! Kinematics and shear senses in Newtonian viscous simple shear zones

Abstract

This work develops an analytical model of shear senses within an inclined ductile simple shear zone with parallel rigid boundaries and incompressible Newtonian viscous rheology. Taking account of gravity that tends to drive the material downdip and a possible pressure gradient that drives it upward along the shear zone, it is shown that (i) contradictory shear senses develop within two sub-zones even as a result of a single simple shear deformation; (ii) the highest velocity and least shear strain develop along the contact between the two sub-zones of reverse shear; (iii) for a uniform shear sense of the boundaries, a zone of reverse shear may develop within the top of the shear zone if the pressure gradient dominates the gravity component; otherwise it forms near the bottom boundary; (iv-a) a 'pivot' defined by the intersection between the velocity profile and the initial marker position distinguishes two sub-zones of opposite movement directions (not shear sense); (iv-b) a pivot inside any non-horizontal shear zone indicates a part of the zone that extrudes while the other subducts simultaneously; (v) the same shear sense develops: (v-a) when under a uniform shear of the boundaries, the shear zone remains horizontal and the pressure gradient vanishes; or alternatively (v-b) if the shear zone is inclined but the gravity component counterbalances the pressure gradient. Zones with shear sense reversal need to be reinterpreted since a pro-sheared sub-zone can retro-shear if the flow parameters change their magnitudes even though the same shear sense along the boundaries is maintained.

Arc-parallel compression in the NW Himalaya: Evidence from structural and palaeostress studies of brittle deformation from the clasts of the Upper Siwalik, Uttarakhand, India

“…The Siwalik rocks, which lie between the MFT and the MBT, are folded into the Mohand anticline, the Dun syncline and the Santaurgarh anticline (Wesnousky et al 1999;Thakur and Pandey 2004). The Mohand anticline is a fault bend fold, developed on the MFT hanging wall (Thakur et al 2007) showing a normal drag (Mukherjee 2012(Mukherjee , 2014. The back-limb of the Mohand anticline dips gently towards NE and the steeper forelimb towards the SW.…”

Subsurface profiling of granite pluton using microtremor method: southern Aravalli, Gujarat, India

“…Oval structure along the E margin represents the Narukot dome with N95° axial trace. Dotted line across the dome and further W shows location of stations (1-32) for microtremor measurements fabric (Passchier and Trouw 2005;Mukherjee 2011aMukherjee , 2012Mukherjee , 2013aMukherjee , b, 2014Mukherjee , 2015Mukherjee and Kovi 2010a,b). Fieldwork did not reveal any visual effect of shear heating (Mukherjee and Mulchrone 2013; Mulchrone and Mukherjee 2015, 2016) The S-and C-planes meet at ~24°.…”

Tectonic Inheritance in Continental Rifts and Passive Margins

“…ductile shear zones (Mukherjee 2012;Mukherjee and Biswas 2014) surrounding rocks (Morley 1999b). ductile shear zones (Mukherjee 2012;Mukherjee and Biswas 2014) surrounding rocks (Morley 1999b).…”